Every now and then, I hear people make comments like "Movies more than 5 years old won't look any better in HD because they were shot before HD was invented – the movie studios are lying to you about better quality!"
This is horribly, horribly, horribly wrong, and I'm not quite sure where this idea stems from.
A "Full HD" image (as the marketing people like to call it) is 1920 pixels wide by 1080 pixels high. Now that sounds like an awful lot of pixels (especially compared to Standard Definition) but it's small compared to the "resolution" of a good film stock.
Although film doesn't have well-defined "pixels" like digital video does – it is based on tiny particles inside the film's chemical structure – you can get an approximation to digital pixels and the current thinking is that it is around 6- to 8- thousand pixels horizontally (not vertically) – something the marketing folks would probably call "Ultimate HD 4320p" or some other silly name.
This brings up an interesting difference in how resolution is counted – in the consumer world, resolution is counted vertically – 480, 720, and 1080 all refer to the number of rows of pixels. But in the digital cinema world, it is counted horizontally – 2K and 4K refer to the number of columns of pixels. So your "1080p" TV is pretty close to a "2K" system in the digital cinema world (although not quite).
What this means is that pretty much any movie or TV show shot on a 35mm (or larger) film stock has far more resolution than even "Full HD" can show, and so old movies still get a big improvement by going to HD DVD (or Blu-ray) versus standard DVD. It all depends on the quality of the master and the time and effort taken in transferring it from film to digital and then compressing it for release.
You want proof? Look at the review of Casablanca (1942) for an example of how old films can look simply stunning in high definition.
Another thing to consider is that despite the fact that film has more resolution than "Full HD," there are lots of problems with the way film is typically viewed that can dampen the experience. Prints get scratched. Projectors get out of focus. Screens get dirty. Theatres dim the light bulbs to save money. All of these things can lead to a poor-quality film presentation, so with a good HD transfer and a well-calibrated display you might even see more on an HD DVD disc than you would in the average supermall Cineplex.